That's because, as far as I recall, the F3 screen can only be trusted with mission time, max altitude and max speed over land. If you look at my submission, the F3 screen says I have traveled only 1,099,219 m ground distance, and I can assure you and prove that I went the whole way. There are problems with the distance traveled measurements everywhere, here, for example, the OP had to completely change the rules and wipe the scoreboard because of it. Also, I think you meant circumference, not diameter